1. Field of the Invention
The present invention relates to a method of connecting an external circuit of a liquid crystal panel, and more particularly to a connecting method suited for connecting an external circuit for a ferroelectric liquid crystal panel, and a packaging structure thereby formed.
2. Related Background Art
Hitherto a method has been developed for mutually connecting FPC (a flexible printed-circuit substrate) and wiring substrates (glass epoxy substrates, glass substrates, ceramic substrates, FPC, etc.) in which they are connected by thermocompression bonding of a film-shaped anisotropic conductive film comprising a conductive material incorporated by dispersion in an insulating resin. Particularly in recent years, in the field of image display units, such as liquid crystal display devices, and EL display devices that can replace conventional CRT, or in the field of image reading units such as close contact image sensors of continuous length integrally formed using amorphous-Si or the like as sensors that can replace conventional CCD, the connecting method using the anisotropic conductive film having high-density resolution is widely used.
Also frequently used as FPC are film carrier tapes on which IC chips can be directly mounted. These film carrier tapes have a wiring function for making connection to image display devices or image reading devices and also have a carrier function that enables achievement of rationale assembly processes.
However, in the display panels connected with external drive circuits according to a conventional connecting method, the resistance becomes so large that a large voltage is required for driving liquid crystal panels.
FIG. 9 illustrates a cross section of a conventional film carrier packaging structure 50, wherein a copper foil 53 constituting a conductive material pattern is bonded on a flexible insulating film 51 with the use of a bonding material 52. At the inside of a device hole 55 bored in the flexible insulating film 51, an inner lead-bonding area 56 is formed in the shape of a finger to make a connection with a semiconductor device 54, at which the semiconductor device is connected.
This film carrier 50 is joined by thermocompression bonding on a transparent conductive film 59 of ITO (indium-tin oxide) or the like, formed, for example, on a glass substrate of a liquid crystal display device, with the use of an anisotropic conductive film 57 comprising conductive particles such as metal particles dispersed in an adhesive.
However, the above conventional film carrier packaging structure has the problems discussed below.
(a) In recent years, liquid crystal display devices have come to be used as large screen displays replacing conventional CRTs, and liquid crystal displays of, for example, 640.times.400 dots or more have come to be used in personal computers and word processors. In addition, there are increasing demands for making screens larger, for increasing the degree of precision and for generating colored images.
In instances where driving ICs are connected to these large screen liquid crystal displays, a packaging structure packaged by the anisotropic conductive film by sue of the film carrier system as mentioned above has recently come to be often used. Known anisotropic conductive film are, for example, CP-2132 (a resin composition comprising a styrene-butadiene copolymer and solder particles disposed therein) available from Sony Chemicals Corp., AC5052 (a resin composition comprising a styrene-butadiene copolymer and Au-plated resin particles disposed therein) available from Hitachi Chemical Co., Ltd., and these can have a connection resolution amounting to 5 lines/mm (200 .mu.m pitch).
Now, in addition to the demands for making screens larger, increasing the degree of precision higher and generating colored images, there are also demands for increasing the density and narrowing the pitch of picture elements of the liquid crystal displays. In making screens larger, a delay in signals from the driving ICs may occur owing to an increase in load impedance of the transparent conductive film on the glass substrate of the liquid crystal display, resulting in a lowering of display quality levels. In order to prevent such a lowering of display quality levels, a system has been devised in which, as shown in FIG. 10, the patterns of the transparent conductive film 59 are drawn from the central part of a screen to both sides of the glass substrate 58, and the film carriers 50 for the driving ICs are connected to both sides thereof. There are also demands for making the pitches of picture elements narrower, for example, 8 lines/mm (125 .mu.m pitch) or 10 lines/mm (100 .mu.m pitch), in regard to making a degree of precision higher and generating colored images, but, as mentioned above, there is at the present time a limit of about 5 lines/mm (200 .mu.m pitch) in packaging film carriers with the use of the anisotropic conductive films. Therefore, for example, as shown in FIG. 11, the patterns of the transparent conductive film 59 on the glass substrate 58 are alternately drawn to both sides of the glass substrate 58 in a zigzag fashion to reduce the pattern pitch to a half, and the film carriers 50 for driving ICs are connected to both sides thereof.
However, when it is attempted to package the driving ICs on both sides of the glass substrate 58, as mentioned above, the following problem occurs. Assume that, as shown in FIG. 12, the liquid crystal picture element driving output of a driving IC 71 to be packaged at one side (the left-hand side in the drawing) is designed to be scanned from 1 to the direction of an arrow as shown in the drawing. When an identical driving IC 71 is packaged at the other side (the right-hand side in the drawing) as shown in FIG. 13, the direction of the liquid crystal picture element driving output of the driving IC 71 becomes as shown in an arrow in the drawing, resulting in reversed scanning directions on both sides of the glass substrate 58 to make it impossible to use the same driving ICs. For this reason, it becomes necessary to design the driving ICs 71 so as to have an output capable of being scanned in both directions, or to use two types of quite different driving ICs (it follows that four types are required since in a liquid crystal display the driving ICs are required respectively for upper and lower two sheets of glass substrates), thus increasing the cost for the driving ICs and complicating the carrier-packaging processes.
The following problems also occur: If the above-described manner of packaging as shown in FIG. 9 is used, a high temperature of from 150.degree. to 250.degree. C. is applied to the flexible insulating film 51 of polyimide or the like formed by hot processing for the thermocompression bonding when the anisotropic conductive film 57 is thermocompression bonded. For this reason the flexible insulating film 51 undergoes, owing to the difference in thermal expansion coefficient between the flexible insulating film 51 and glass substrate 58, thermal expansion at the time of the thermocompression bonding. As a result, film 51 shrinks after the thermocompression bonding. This causes slippage between the film 51 and the glass substrate 58. Such slippage is generated in the anisotropic conductive film 57 or a stress is applied thereto to bring about an increase in connection resistance and a lowering of connection strength, resulting in a lowering of reliability in the packaging of film carriers.
(c) Additionally, the following problems also occur: at the time of the thermocompression bonding, it is actually desirable to apply heat to the anisotropic conductive film in the range of from 130.degree. to 180.degree. C. (variable depending on manufacturer's products) with the precision of about .+-. 5.degree. C. However, in the convention packaging structure heating is carried out by means of a hot press for the thermocompression bonding through the flexible insulating film 51 of the film carrier 50. As a result, this flexible insulating film may work as an thermal insulating material, so that it is difficult to control the thermal energy to be applied to the anisotropic conductive film; also heating is carried out for a long time.